In machinery it is often necessary to ensure retention of a screw-threaded nut to avoid loosening in service, for example due to machine vibration. In a safety critical application, mechanical locking techniques include the use of locking wire and tab washers. In the latter case a washer is placed under the nut and includes both an anti-rotation element secured on a component other than the nut, and a tab adapted to be bent into engagement with the nut when in the fully tightened condition. Tab washers are single use items and, if correctly fitted, are highly reliable.
A tab washer may constitute a primary mechanical lock for the nut. In some circumstances it is necessary to provide a secondary mechanical lock for the nut. The secondary mechanical lock should preferably be available to be fitted only if a secondary lock is desirable, and furthermore should be retrofittable in cases where the addition of a secondary lock is considered to be necessary.